Pearls & Oy-sters: Recurrent Lyme Neuroborreliosis With Seroreversion in a Patient With Multiple Sclerosis on a B-Cell Depleting Therapy.

True seronegativity is extremely rare in Lyme neuroborreliosis (LNB) with reports only in patients with hematological malignancies or under treatment with chemotherapy and B-cell depleting therapies. In these instances, diagnosing LNB can be challenging. We report the case of a 63-year-old patient with 2 independent episodes of LNB. During the first episode with lymphocytic meningitis, anti-borrelial IgG and IgM were detected in serum and CSF. However, initial seropositivity converted to seronegative serum at 8 months of follow-up and remained seronegative during a second episode of LNB while on B-cell depleting treatment for multiple sclerosis. During this second episode, the patient reported painful meningoradiculoneuritis (Bannwarth syndrome), yet no anti-borrelial antibodies could be detected in serum or CSF. Borrelial PCR was positive in CSF, leading to the diagnosis of LNB. Symptoms resolved after antibiotic treatment. Cases of seronegative LNB can occur in the context of B-cell depleting agents. Standard antibiotic treatment is successful for LNB in the context of immunosuppressive treatment. Further diagnostic investigations with PCR or CXCL13 should be considered in cases with high clinical suspicion.